Influence of martensite formed during deformation on the mechanical behavior of Fe-Ni-C Alloys
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A. G O L D B E R G ,
A N D J. C. S H Y N E
Tension t e s t s were p e r f o r m e d on m e t a s t a b l e austenitic F e - N i - C a l l o y s to study the influence of m a r t e n s i t e f o r m e d during s t r a i n i n g on mechanical b e h a v i o r . Although both s t r e s s a s s i s t e d , plate m a r t e n s i t e and fine, lathlike s t r a i n - i n d u c e d m a r t e n s i t e were f o r m e d , only s t r e s s - a s s i s t e d m a r t e n s i t e o c c u r r e d in sufficient amounts to influence mechanical b e h a v ior. The f o r m a t i o n of s t r e s s - a s s i s t e d , plate m a r t e n s i t e r a i s e d the flow s t r e s s and the s t r a i n - h a r d e n i n g r a t e , but only after 25 to 40 pct m a r t e n s i t e had formed. A TRIP effect was obtained, i . e . , ductility was enhanced by c o n c u r r e n t m a r t e n s i t e formation, but this o c c u r r e d over only a n a r r o w t e m p e r a t u r e range b e c a u s e of the s t r o n g t e m p e r a t u r e dependence of the a u s t e n i t e - t o - p l a t e m a r t e n s i t e r e a c t i o n . This is in c o n t r a s t to the b e h a v i o r of h l g h - C r austenitic s t e e l s , which exhibit a TRIP effect over a wide t e m p e r a t u r e range b e cause of the f o r m a t i o n of l a r g e amounts of fine s t r a i n - i n d u c e d m a r t e n s i t e , whose f o r m a tion is l e s s t e m p e r a t u r e - s e n s i t i v e than that of plate m a r t e n s i t e .
M E T A S T A B L E austenitic F e - N i - C a l l o y s t r a n s f o r m to m a r t e n s i t e during p l a s t i c deformation. The influence of e l a s t i c s t r e s s and p l a s t i c s t r a i n on the a u s t e n i t e - t o - m a r t e n s i t e t r a n s f o r m a t i o n in such alloys is d i s c u s s e d in the p r e c e d i n g p a p e r . ~ The p r e s e n t paper d e s c r i b e s the c o n v e r s e effect, that is, the influence of m a r t e n s i t e f o r m e d during mechanical testing on the o b s e r v e d mechanical b e h a v i o r of m e t a s t a b l e austenitic Fe-NI-C steels. That martensfte formation can occur during plastic deformation has long been recognized, ~ and it is known that mechanically induced martensite can enhance subsequently measured mechanical properties, s-*z Also, it has been demonstrated that when martensite forms during plastic deformation advantageous concurrent mechanical behavior results. *~ Most notably, Zackay and co-workers zs-s6 have utilized the transformationinduced plasticity of their TRIP steels to develop the excellent ductility of these high-strength metastable austenitic steels. Quite evidently, the high ductility of the T R I P steels is caused by the formation of martensite during plastic deformation. The present study was intended to elucidate the T R I P effect by observing its occurrence in metastable Fe-NIC alloys. The experimental materials were chosen for their simplicity compared to the compositionally and microstructurally complex T R I P steels. The preceding pap
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